The phytohormone, abscisic acid (ABA) plays critical roles in physiological processes including seed dormancy, germination and adaptive responses to environmental stresses (Finkelstein R. R. et al., Plant Cell 14: 515, 2002; J. K. Zhu. Annu Rev Plant Biol. 53: 247, 2002; Zeevaart J. A. D. and Creelman R. A., Ann. Rev. Plant Physiol. Plant Mol. Biol. 39: 439, 1988; Leung J. and Giraudat J. Annu Rev Plant Physiol Plant Mol Biol. 49: 199, 1998). ABA was found almost at the same time by two study groups; F. T. Addincott and P. F. Wareing. F. T. Addincott group, studying on the falling of unripe fruits, isolated abscis II as a stimulator of dropping leaves from cotton fruits. And the other study group of P. F. Wareing, studying on winter dormancy of tree buds, isolated an inducer of dormancy from Betula Pubescence, a member of birch, and then named it dormin. In 1965, dormin and abscis II were confirmed to be identical, then, they began to be called abscisic acid (referred as ‘ABA’ hereinafter).
Dormant seeds, tree buds and bulbs contain a huge amount of ABA and the content of ABA decreases as they are germinated. ABA is also involved in stomatal movement. Precisely, when a plant is dehydrated, ABA synthesis is accelerated and pores of a leaf are closed thereby, resulting in the protection a plant from water loss. For example, a transgenic tomato plant named flacca showed no stomatal closure because of low levels of ABA, compared with that in a wild type plant.
Cellular ABA levels fluctuate constantly to allow plants to adjust to the changing physiological and environmental conditions. In particular, cellular ABA levels are controlled by equilibrium between biosynthesis and degradation. De novo protein synthesis is essential to increase the cellular ABA level. In the meantime, ABA level decreases by degradation of ABA resulted from oxidation or conjugation into inactive forms (Zeevaart J. A. D. and R. A. Creelman, Ann. Rev. Plant Physiol. Plant Mol. Biol., 39: 439, 1988; Leung J. and Giraudat J., Annu Rev Plant Physiol Plant Mol Biol. 49: 199, 1998; Cutler A. J. and Krochko J. E., Trends in Plant Science 4: 472, 1999; Qin X. and Zeevaart J. A., Proc. Natl. Acad. Sci. USA. 96: 15354, 1999).
Low levels of ABA result in a variety of physiological defects including precocious germination, wilting and sensitivity to environmental stress (Cutler A. J. and Krochko J. E., Trends in Plant Science 4: 472, 1999; Koornneef M. et al., Theor. Appl. Genet. 61: 385, 1982; Rock C. D. and Zeevaart J. A., Proc. Natl. Acad. Sci. USA. 88: 7496, 1991), which indicates that controlling ABA levels properly by equilibrium between synthesis and degradation is very important for various physiological responses. The cellular ABA content is lowered via two pathways. The first pathway is hydroxylation of ABA at the 8′ position by cytochrome P470 CYC707A to form unstable 8′-hydroxy ABA. This unstable intermediate is subsequently converted to phaseic acid by spontaneous isomerization (Zeevaart J. A. D. and Creelman R. A., Ann. Rev. Plant Physiol. 39; 439, 1988; T. Kushiro, et al., EMBO J. 23: 1647, 2000)
The other pathway lowering cellular ABA content is conjugation with glucose which is mediated by glucosyltransferase, thereby ABA glucose ester (ABA-GE) is generated (Cutler A. J. and Krochko J. E., Trends in Plant Science 4: 472, 1999; Walton D. C. and Li Y., in Plant Hormones: Physiology, Biochemistry and Molecular Biology, 140-157; Xu Z. J. et al., Plant Physiol. 129: 1285, 2002). Conjugated ABA-GEs are stored in vacuoles or apoplastic space (Kaiser W. et al., J. Plant Physiology 119: 237, 1985; Dietz K. J. et al., J. Exp. Botany 51: 937, 2000). However, the issue of whether biologically inactive ABA-GEs constitute a reserved or stored form of ABA remains to be clarified.
In the present invention, the inventors confirmed that AtBG1, an Arabidopsis β-glucosidase homolog localized to the ER, displays ABA-GE hydrolyzing activity, and increases the cellular ABA content through rapid polymerization of lower molecular weight AtBG1 into higher molecular weight forms under conditions of dehydration stress. A mutant Arabidopsis thaliana (referred as ‘mutant atbg1’ hereinafter) with a T-DNA insertion in AtBG1 displayed a reduced ABA level, defective stomatal closure, a yellow leaf phenotype due to the lack of chloroplast and sensitive response to abiotic stress. On the other hand, a mutant Arabidopsis thaliana over-expressing the AtBG1 gene displayed 2.5-fold higher ABA level and stronger resistance to environmental stress than those of control.
The present inventors, therefore, completed this invention by confirming that the AtBG1 gene can be a great asset to a plant having a strong resistance to various environmental stresses including salt damage, cold damage and dehydration.